According to portfolio theory, people should hold more than one asset (or investment) in their portfolios

The claim that not everyone will diversify is TRUE. The most famous person who practice this is in fact our famous investor WARREN BUFFET.

Warren once said: "Diversification is a protection against ignorance. It makes very little sense for those who know what they are doing."

Many financial advisors recommend that you diversify for your own protection. What they fail to tell you is that it is also for their protection. Since most financial advisors cannot tell you exactly which stock or mutual fund is a great investment, they tell you to buy a bunch of them.

In fact the clearest example is the company Berkshire Hathaway which Warren held from years to years. It is the most successful story that you can put in your work. This is an example of if you really know what you're doing and that you'll be successful, why diversify?

Have a look at the references for more information. But always remember that these are only personal investment philosophy.

http://finance.yahoo.com/columnist/article/richricher/1649
http://www.woopidoo.com/business_quotes/authors/warren-buffett-quotes.htm